1. Technical Field
The present invention relates in general to the field of a photoelectrode, a method of fabricating the same, and applications of the photoelectrode, and more specifically to a photoelectrode including a zinc oxide (ZnO) hemisphere, a method of fabricating the same, and a dye-sensitized solar cell (DSSC) using the same.
2. Related Art
Since the pioneering work of O'Regan and Grätzel in 1991, numerous research studies have investigated dye-sensitized solar cells (DSSCs) as an alternative, next generation solar cell. This evolution has continued to progress, and solar light-to-electricity conversion efficiency (power conversion efficiency (PCE)) has now exceeded 11%. DSSCs have recently garnered increasing attention as an ideal photovoltaic concept; the advantages of DSSCs are low-cost, transparency, color rendition, eco-friendly process, bio-compatibility and simplicity. Generally, improvements in overall PCE have focused on increasing photovoltage through modification of an oxide layer, improving photocurrent with new dye molecules, developing new electrolytes, and increasing stability by cell configurations.
A transparent mesoporous titanium dioxide (TiO2) nanoparticulate layer is a well-known photoelectrode (photoanode) material used in conventional DSSCs. However, the small TiO2 nanoparticulate layer with a diameter of about 20 nm, which is transparent to visible light, weakly scatters light due to the small particle size. As a result, a substantial portion of incident light passes through the TiO2 nanoparticulate layer without being captured and utilized to produce photo-current.
Many studies have focused on capturing more light in a photoelectrode film using sub-micron poly-dispersed oxide particle aggregates, which act as effective scattering centers, and/or using gradient scattering layers consisting of TiO2 nano-particles with different radii along the path of light. Although the utilization of the larger sized aggregates within the photoelectrode film with a thickness of about 9 μm and a cell area of about 1 cm2 enhances PCE to 5.4%, due to the improved light scattering, the aggregates decrease the total surface area and therefore the dye adsorption to the aggregates is also decreased.
Improvement in PCE of DSSCs is also hindered by energy losses due to the recombination of produced electrons with both oxidized dye molecules and electron-accepting components in an electrolyte during a charge transport process. The recombination problem becomes significant with the thickness of the photoelectrode film. Although the thick film can contain more dyes for enhancing light harvest, it inherently contains cracks and encounters mass transport limitation of a redox electrolyte, thereby reducing the photovoltage of the cell.
To overcome the recombination issue in the thick particulate film, ZnO nanostructure-incorporated photoelectrodes have recently been studied; nano-wires, nano-tubes, or nano-trees. ZnO is a wide bandgap semiconductor that has an energy-band and physical properties similar to those of TiO2. Interestingly, by controlling a growth process, a ZnO crystal structure (wurtzite) enables various morphological changes: nanorod, nanoribbon, nanobelt, nanocomb and so on, during the growth, which is not possible with the crystal structures of TiO2 (either anatase or rutile). Such various nanostructures present their distinguished performances in photovoltaic devices by providing higher electron mobility or larger surface area. One-dimensional nanostructures of ZnO are favorable for fast electron transport with reduced recombination losses by providing direct conduction pathways to the collecting electrode.
However, conventionally reported DSSCs with various ZnO nanostructure-incorporated photoelectrodes have not yielded higher PCE (%) than TiO2-film photoelectrode. In the case of conventional photoelectrodes composed of ZnO nanostructures, the nanostructures are not spatially arranged but inter-connected, and a surface area where dyes can be adsorbed is smaller than that of a nanoparticulate film.
The ZnO crystalline structure is intrinsically weak to an acidic dye solution (pH of ca. 5˜6), which is the crucial drawback for the application of ZnO for DSSCs; the origin of Zn2+/dye aggregation causes a low open-circuit voltage and a poor long-term stability. Therefore, ZnO nanostructure-incorporated photoelectrodes coated with a TiO2 film to provide the fast electron transport of the ZnO nanostructures, the large dye adsorption of the TiO2 film, and a resistance to an acidic dye solution have been explored.
In general, although light is irradiated to the photoelectrode, a TiO2 nanoparticulate film serves as a photo-catalyst to degrade the properties of a dye under UV irradiation. However, when light is irradiated to a catalytic electrode, an electrolyte can absorb most UV light before the light reaches the photoelectrode to prevent degradation of the dye, resulting in long-term stability of device performance.